I believe that no one really knows how much because of the amount of the users of bitcoin. Imagine the guy who throw away his hard drive with tons of BTC in it. Yes that's a real news... One of the main reasons it's basically impossible to exactly know the % of the supply that's lost forever is the fact that you can't know if an address that hasn't got activity for a long time means the owner lost the password, or the owner is simply holding those coins and working with other addresses for transactions.